Exhaust muffler



Feb. 9, 1932. F. H. SCHNELL EXHAUST MUFFLER Original Filed Ma"Mill/kill) NVENTOR E 56/7/72 WW w 634W ATTORNEYS Patented Feb. 9, 1932UNITED STATES FFICE FREDERICK H. SGHNELL, 0F MADISON, WISCONSIN,ASSIGNOR T0 Q. 15. BURGESS LABORA- TORIES, INC., 015 MADISON, WISCONSIN,A CORPORATION 01' DELAWARE EXHAUST MUFFLER Original application filedMay-8, 1929, Serial No. 361,376. Divided and this application filed May29, 1929. Serial No. 366,892.

My invention relates to mufliers for noiseproducing gases and moreparticularly to exhaust mufllers for expanding and noise-producingpulsating gases such as are discharged from internal combustion engines.

This application is a division of my copending application, Serial No.361,37 6 filed May 8, 1929, (now Patent No. 1,811,762, granted June 23,1931).

My improved exhaust mufiler departs very decidedly in construction fromthe exhaustv mufilers heretofore employed.

,The explosion within the cylinder of an internal combustion enginecreates tremendous pressure therein. The steam or vapor in the cylinderof a steam engine is also under very high pressure. Upon the opening ofthe exhaust port, the gas emerges at high velocity, very quicklyreducing the pressure in the cylinder. By the word gas, I mean toembrace both gas and vapor. The result is a succession of pulses of gasat high pressure traveling through the exhaust manifold or pipe or anyother type of exhaust system at a high velocity. Upon the emergence ofthe gas pulse into the atmosphere an additional spurt in velocity isprobably acquired by reason of expansion, and a sharp sound results. Thegreater the pressure gradient between the pulse of gas and theatmosphere the greater the velocity and the sharper the sound. In orderto successfully silence exhaust noises the muffler must suppress thepressure peaks and thus even the flow of the escaping gas pulses to thepoint where a sound is not created. In addition, I have found that soundcommunicated to or generated within the muflier should be silenced.

The mufflers of the present day accomplish the desired result by any orall of the following methods: cooling the exhaust gases and diminishingtheir volume; allowing the gases to expand and reducing their'pressurewhile confined; creating eddy currents and internal friction within thegases; causing friction between the gases and the walls and passageswithin the muflier; and/or impeding the forward progress of the gases bythe interposition of baflies in their path. The typical muflier oftodayis a metal shell connected in the exhaust system of an engine whichmay incorporate one or more of the following features: an expansion andmixing chamber followed by a small outlet; a tortuous passageway; aseries of obstacles or baflies in the passageway; and/or a jacket inwhich a cooling medium is circulated.

Dnly a limited sphere of application can at best be found for amuflierwith a cooling jacket. The expansion chamber alone is not successfulsince it approaches the condition of the atmosphere and sound is createdtherein. The types embodying tortuous paths, obstacles, and baffles,retard the flow of gas, and in so doing, build up a back pressure withinthe muflier and reduce the efficiency of the engine. There is a mufflerbeing produced which actually assists in the scavenging of the cylinderbut the scope of its application is limited to special types of enginesof known, constant speed.

It is an object of my invention to provide an exhaust muffler whosescope of application extends to practically all of the common types ofexplosion and steam engines, to fire arms, and to air intakes andexhausts as in air compressors.

It is a further object of my invention to provide a mulher which willnot build up sufficient back pressure to appreciably reduce theefliciency of the engine.

It is a further object of my invention to provide a mufiler which willsilence explosion noises more elfectively than do the mufilers atpresent employed.

It is a still further object of my invention to provide a muffler whichis more simple in construction than are the present mufflers ofcommerce.

Uther and further objects of my invention will become apparent as thefollowing description progresses, which is to be taken in conjunctionwith the accompanying drawings, wherein:

Fig. l is a longitudinal, sectional view of one form of the invention;and

Figs. 2 and 3 are charts which illustrate the effect of my mufiiingdevice upon exhaust gases.

In the form of mufier for internal combus- .tion engines shown in Fig. 1of the drawtake and exhaust noises, and noises produced by gases at roomtemperature, flammable gaspressure absorbing and sound-absorbingmaterials such as wool, cotton, or other cellulosic fibers may be used.

In view of the fact that every material which is not a perfect reflectorof sound is, to a degree, an absorber of sound, it would probably bewell to draw some sort of a specification for the term sound-absorbingmaterial as that term is used in the present case. In all acoustic datathe sound transmitted by an open window is used as the standard ofcomparison. The ratio of the sound absorbed by an area of material tothat transmitted by an equal area of open window is called theabsorption factor or value of that material. If a material one squarefoot in area absorbs one-fourth the sound transmitted by one square footof open w-idow,-that material is said to have an absorption factor orvalue of 25 percent. In this specification whenever the termsoundabsorption factor or value is used in conjunction with an absorbingmaterialor construction in a muffler, it is tobe understood that thisfactor or value is obtained by measuring the absorption of such materialor construction in the usual Way by means of fiat r pad-s and the likeof identical material and construction. This is necessary since it isvery difiicult or impossible to obtain the factor after'th'e mufller isassembled. In the mufflers of my invent-ion as the sound absorptionfactor of the sound absorbing material decreases, the size of themufller must increase and a material with a factor of 10 percent or lesswould probably necessitate an unwieldly size. Hence, in the presentconsideration, by sound-absorbing material I do not contemplate amaterial having an absorption factor of less than 10 percent at 1024double vibrations per second. .Throughout this specification allabsorption values are based on 1024 double vibrations per second. Formost situations an excellent muffler may be constructed with a materialhaving The absorbing material should be of appreciable thickness forbest results. It should be at least one-quarter of an inch thick andpreferabl one-half inch or more. By mineral fibre contemplate natural orartificial mineral wool, shredded asbestos, or any other mineralmaterial of the same general nature.

' An inner cylinder 3 of foraminous rigid material such as perforatedsheet metal provides a straight, open, unobstructed central duct 4extending from -end to end of the muffier. Because of the hightemperature of in,-

ternal combustion engine exhaust gases, cylinder 3 expands and thereforeit preferably is not fastened to either metal disc 5 or 6 or both,thereby forming a slip joint and preventing buckling, since cylinderlwhich is also fastened to discs 5 and 6 does not expand. Disc 5 may beprovided with a threaded central opening to receive pipe 7 whichconducts the exhaust gases into the mutfler. It is usual in automobilemufflers to provide a slip joint so that pipe 7 is free to move in disc5 and such construction may be employed in place of the threadedconnection shown. The arrows in the drawings illustrate the direction ofgas flow. The gases are finally exhausted through pipe 9 in opening 8 ofdisc 6. It is preferred to have the internal diameter of cylinder 30fthe same size as exhaust pipe 7. If it is smaller it lncreases the backpressure. As will be discussed further hereinafter, if cylinder 3 islarger than pipe 7 the acoustic absorption decreases.

As shown the larger end of the truncated cone shell 1 is arranged at theinlet end of the mufiier. As the gas pressure and sound absorbingmaterial 2 fills the space between this shell and inner cylinder 2, itdeer-eases ifln thickness toward the outlet end of the muf- Ashereinbefore explained, a, series of gas pressure peaks emerges at highvelocity from pipe 7. Each peak tends to expand in all directions. Thegas and pressure waves freely pass. through perforated cylinder 3 andenter annular space 2. The property of the perforated sheet metal ofpermitting the free transmission of gas and sound therethrough, togetherwith its other advantages, makes it the preferred foraminous materialfor a device of this character. The perforations may vary in size andshape. Because of low- 1ngs.-- Holes suitable may vary fromapproximately 0.070 to 0.125 inches in diameter but 1 do not wish tobelimited to these dimensions. For most purposes the holes are evenlydistributed throughout the area of the sheet metal although such evendistribution is not necessary. The holes are present in sufficientnumber so that their total area represents from 2 percent up to 35percent of the area of the sheetmetal though fair results may beobtained with holes having an area as low as 1 percent of the area ofthe sheet. The shape, size and distribution of the perforations all havean influence on the ease with which gases and sound pass through thesheet of metal. The holes should be small enough so that the absorbingmaterial will not work its way out through them when the muffler is inservice. Excellent results are obtained when the ratio of theunperforated portion of the metal forming such facing to the openingstherein is such that a substantially continuous surface is exposed tothe sound waves and gas pressure waves. In such cases the averagedimensions of the individual openings are usually less than the distancebetween the edges thereof. Tests show that a muffler containingabsorbing material faced with a stiff perforated sheet, the perforatedarea of which is as low as 2 percent of the area of the sheet with holesabout .075 inch in diameter, absorbs as much sound as and has a mufiiingefliciency equal to a similar muffler in which the perforated facing isomitted. It is possible to omit the perforated metal facing or otherforaminous facing by construct-ing a molded annular or other suitablyshaped absorbing material as by bonding sized crushed mineral particlestogether at their points of contact to form a porous mass. Such a moldedgas-pressure absorbing and sound absorbing material has an openingtherethrough corresponding to the foraminous tube 3. In anotherconstruction metallic fibers, like steel wool, are formed into amattress by stitching the fibres together with a metallic thread such assteel wire. This mattress is used to line the inside of the mufflershell 1. The foraminous duct may be omitted when such a mattress isused.

The fibrous gas-pressure absorbing and sound absorbing materialcomprises nonflammable fibres distributed in heterogeneous arrangementto form tiny interstices, pores orcells of more or less uniform size anddistribution. It is packed in such a manner that usually only from 1 to20 percent of annular space 2 is occupied by the actual fibres of thematerial and therefore 80 to 99 percent is free space. The packedfibres, depending on their physical properties, weigh after packing inthe muffler, from 4 to 100 pounds per cubic foot. The packed mineralwool weighs from 9 to 36 pounds per cubic foot, the fibres occupyingfrom 5 to 20 percent of the space. Steel wool weighs from 4 to 100pounds per cubic foot, the fibres occupying from 1 to 20 percent of thespace. Especially good results are obtained with steel wool packed tooccupy 2 to 5 percent of the space. If the fibres are packed too looselythey jar down into a more compact mass after the muffler is put intouse, and if they are packed too tightly the gas-pressure absorbing andacoustic absorption are cut down, thereby decreasing the muflierefficiency. The absorbing material is subjected to violent pounding andvibration by the oscillating influence of the exhaust gases and must notdisintegrate readily under those conditions. Certain types of mineralwool resist this disintegrating action much better than others.

However, a metallic wool like steel wool is highly resistant to thisaction. The soundabsorbing portion of the mufller, therefore, providesan expansion space for gas-pressure peaks and the maximum pressure ofthe pulse of gas is decreased as a result of the expansion, the pressurewave being longer and lower in intensity. Fig. 2 shows in solid lines ahypothetical diagrammatic representation of several gas pulses as theyenter the muflier and in dotted lines is a similar representation of thesame pulses after expansion has taken place. The pressure wavecharacteristic has become flattened. I also believe that a sec- 0ndphenomenon takes place within annular form of a succession of rapidcompressions and rarefactions. The compressions are suppressed andmerged together by the choking effect of the walls of the tiny pores orcells with a resulting reduction of the intervening rarefactions. Fig. 3shows in solid lines the pressure wave characteristic corresponding tothe dotted curve of Fig. 2. The dotted lines of Fig. 3 show the pressurewave characteristic after the gas has undergone the choking action ofthe porous sound-absorbing material. The porous material offerspractically no resistance to the fairly steady flow of the gas streamthrough the straight and unobstructed duct 4, and especially if duct 4is made of smooth perforated, sheet metal. it facilitates the flow withthe net result that there is practically no resistance to the gas flowand little or no back pressure is built up aside from that due to thesurface friction. The absorption of the gas-pressure peaks by theabsorbing material to smooth out the gas flow and eliminate the noisemade by the gas slugs when emerging into the atmosphere is an importantfeature of my invention. I use the sound-absorbing material for thisdouble function without appreciably increasing the of the mufiler withinreasonable limits produced no appreciable change in ultimate quantity ofsound, measured by means of an acoustimeter, WlllCll emerged from themuffler when the latter was connected in the ordinary manner in theexhaust system of an internal combustion engine. However, when theunmufiled exhaust noises were acoustically separated from the gases andconducted to the muffler through anintake pipe similar to that used inthe prior tests, it was found that the quantity of sound emerging fromthe muffler varied with the diameter of the mufiier. From such data itappears that a muffler ,of small diameter, while it does not permit muchex pansion and does not act as efliciently upon the gas pulses, actsmore efficiently upon the noise created therein with ultimate mufl'linequal to or better than that of the larger device which allows the gasto expand more and thus acts more efficiently upon it but acts lessefliciently upon the noises created therein. The muffler absorption forany one absorber may be increased by increasing the thickness of theabsorber within limits, or by increasing the outside diameter of themufiier within limits, or by increasing the length of the absorberwithout changing its thickness, or by decreasing the gas duct diameterwithin limits.

lVhile I have illustrated and described but a very few simple forms ofmy invention, it is understood that I may employ a great many formssince I believe that I have inventedthe broad principle of providing amuffier with gas-pressure absorbing and sound absorbing materialadjacent the path 'of the exhaust gases and do not wish to be confinedto structural details. For instance, the inner shells may be of wirescreen, or other suitable foraminous material, or may be entirelyomitted as hereinbefore explained, and the general shape of the deviceneed not be cylindrical. Its cross section may be circular, rectangular,elliptical or of any other form and the manner of assembling the partsmay vary considerably from that described hereinbefore. The rectangularsection is more effective than the circular section because of thegreater area of absorber exposed to the gases in proportion to thecross-sectional area of the duct. My muffler consists essentially of aduct lined with gas-pressure absorbing and sound absorbing constructionwhich ma be a duct lined with absorbing material without a foraminouslining. It may be a fibrous gas-pressure absorbing and sound-absorbing'jectionable.

preciable back pressure. Many present muf flers assist the silencingaction by constricting greatly the cross-sectional area of the gasoutlet but this cannot be done without building up back pressure.

Although published data available relative to mechanics of noiseproduction by internal combustion engine exhausts are meagre, I believethe hypotheses and explanations hereinbefore advanced to explain themanner in which my device accomplishes the muffling of exhaust orexplosion noises are true. Whatever the theories and explanations maybe, the fact remains that, although a substantially straight andunobstructed g path is offered the exhaust gases and noises they do nottraverse that path unchanged but undergo a change therein which causesthem to emerge almost, if not quite, noiseless.

Mufliers which are considered satisfactory and used by automobilemanufacturers, show by acoustimeter measurement, a total mufflingefficiency of at least percent when the internal combustion engines, forwhich the mufflers were designed, are run at full load. A muffler for anautomobile engine which has a mufiling efficiency at full load ofpercent is exceptional. Although the muflling efficiency of satisfactorymufllers for automobiles is only 55 percent at'full load, it must berecognized, that in cities where this efficiency should be high,automobile engines practically are never run at full load. Under lightloads, characteristic of city driving, the efficiency may be Well over95 percent. Even on country roads where high speeds may be attained,automobile engines are seldom subjected to full load. On the other hand,motor boat engines and especially outboard motors, are very often run atfull load, so that unless the muffler efficiency is high, that is. above95 percent, the exhaust noise is excessive. This noise is heard for along distance over the water and therefore is ob- The ordinary outboardmotor muffler, although seemingly inefficient, has an efficiency of fromJ7 5 to 90 percent at the sacrifice of some power due to back pressure.Stationary internal combustion engines and others which are run so thatthey are well loaded, usually must be equipped with high efficiencymufflers.

The efficiency of the muffler is determined by measuring the noisegenerated by the gases escaping into the atmosphere with and withoutmuifiing. The percent efficiency is the ratio of the decrease inmeasured sound to that measured without muffling. 'l he muffierefliciency should be determined when the muffler is used with the enginefor which it is designed. This is essential since the size,

shape, amount of absorbing material, and

other variables of the muffler are determined for each of the varioussizes and types of engines.

Using my invention, as hereinbefore described, it is possible toconstruct mufflers of varying efficiencies, from 55 percent to well over95 percent by varying the quantity and type of absorbing material and byvarying the construction as hereinbefore described. Such mufflers may beconstructed so as to cause practically no increase in the back pressureon the engine at full load aside from that due to the surface frictionof the duct walls. Furthermore, such mufflers occupy a small space, arelight in weight, and may be built at a considerable saving in cost overthe present-day mufiler.

I claim:

1. A muffler for noise producing expanding gas comprising thecombination with a substantially imperforate casing of a duct arrangedin said casing, and forming a substantially straight passage for saidgas through said casing, said duct having a gaspressure absorbing andsound absorbing wall structure, said absorbing structure decreasing inthickness from the inlet to the outlet of said duct and consisting ofporous packed material in layers of appreciable thickness whereby thenoises are effectually silenced.

2. A muflier for noise producing gas comprising the combination with asubstantially imperforate casing, of an open-ended duct arranged in saidcasing and forming a substantially straight passage for the gas throughsaid casing, said duct having a gaspressure absorbing and soundabsorbing wall structure which decreases in thigkness from the inlet tothe out-let of said duct, said wall structure being of such efficiencythat its average sound absorbing value is at least 25 per cent.

3. An exhaust muffler for internal combustion engines comprising aninner, open-ended, perforated, cylindrical, metal shell and an outerco-axial open-ended truncated cone shaped substantially imperforatemetal shell surrounding said inner shell and spaced therefrom, thelarger end of the cone being toward the intake end, the annular spacebetween said shells being filled with porous packed metallic fiber,

4. A muffler for the exhaust gas of an internal combustion enginecomprising the combination with a substantially imperforate casing, of acylindrical duct arranged in said casing and forming a passage for thegas through said casing, said duct having a gaspressure absorbing andsound absorbing wall structure comprising porous packed nonflammablefibers, said wall structure decreasing in thickness from the inlet tothe outlet of said duct, said muffler having a muliling efficiency of atleast 55 per cent when used with a fully loaded engine.

5. An exhaust mufller for internal combustion engines comprising aninner, openended, perforated metal cylinder, an outer coaxial,open-ended, truncated cone shaped substantially imperforate metal shellsurrounding said cylinder and forming an annular space between saidcylinder and said shell, the larger end of the cone being toward theintake end, and a filling of porous packed non-flammable fibers arrangedin the annular space between said cylinder and said shell. said fiberspresenting surfaces to the exhaust gas passing through the muffler andbeing of such efiiciency as a gas-pressure absorbing and sound absorbingmaterial that the average sound absorbing value is at least 25 per cent.

6. A mufiler for noise producing gas comprising the combination with asubstantially imperforate casing, of a foraminous duct arranged in saidcasing and forming a passage for said gas through said casing, said ducthaving a gas-pressure absorbing and sound absorbing backing, saidbacking decreasing in thickness from the inlet to the outlet of saidduct, said backing comprising porous packed non-flammable fibers, theindividual fibers occupying from 1 to 20 per cent of the space occupiedby said packed fibers.

7. A muffler for the exhaust gas of an internal combustion enginecomprising the combination with a substantially imperforate casing, of aforaminous duct arranged in said casing and forming a substantiallystraight passage for the gas through said casing, said duct having agaspressure absorbing and sound absorbing backing, said backingdecreasing in thickness from the inlet to the out let of said duct.

8. A muffler for the exhaust gas of an internal combustion enginecomprising the combination with a substantially imperforate casing, of aperforated metal duct arranged in said casing and forming asubstantially straight passage for the gas through said casing, saidduct having a gas-pressure absorbing and sound absorbing backing, saidbacking decreasing in thickness from the inlet to the outlet of saidduct, said backing comprising porous packed steel wool, the fibersthereof occupying from 1 to 20 per cent of the space occupied by saidpacked steel wool.

9. A muffler for the exhaust gas of an in ternal combustion enginecomprising the combination with a substantially imperforate casing. of aforaminous duct arranged in the casing and forming a substantiallystraight passage for the gas through said casing,-and

a gas-pressure absorbing and sound absorbing backing for said duct, saidbacking varying in thickness.

10. A mufiier for noise producing gas comprising the combination with asubstantially imperforate casing, of a duct arranged in said casing andforming a substanitally straight passage for said gas through said 1casing, said duct having a gas-pressure absorbing and sound absorbingwall structure, said absorbing structure decreasing in thick ness fromthe inlet to the outlet of said duct and consisting of porous packedmaterial in layers of appreciable thickness, said material weighing from4 to 100 pounds per cubic foot.

11. A muflier for theexhaust gas of an internal combustion enginecomprising the combination with a substantially imperforate casing, of aduct arranged in said casing and forming a substantially straightpassage for the gas through said casing, said duct having a gas-pressureabsorbing and sound absorbing wall structure, said structure varying inthickness and weighing from 4 to 100 pounds per cubic foot.

In testimony whereof I afiix my signature.

' FREDERICK H. SCHNELL.

